Consider the following image:
From left to right: 50mm f/1.8D, 50mm f/1.8G, 50mm f/1.4G. Notice anything? Yep, the new f/1.8 is basically the same size as the f/1.4: you're not gaining any substantive size advantage in opting for the f/1.8G over the f/1.4G. So just what advantage do you gain? Preliminary testing says that the f/1.8G is in most optical respects slightly better than the f/1.4G. The f/1.8G is also significantly lighter than the f/1.4G and less than half the price. The f/1.4G's advantage? It's two-thirds of a stop faster. If you're trying to stay compact (say, trying to build a D3100/D5100 small prime kit), the new 50mm f/1.8G is a bit of a disappointment: adding AF-S nets you a 10%+ gain in length and a 20% gain in weight while taking another US$90 out of your pocket. It's likely that the f/1.8D is no longer in production, so if you want a truly small 50mm from Nikon, get the f/1.8D while you still can. But remember, it doesn't autofocus on the low end Nikon DSLR bodies.